Circuit for periodic introduction of electrons into an electron accelerator



March 2 7, 1951 2,546,484

R. WIDEROE CIRCUIT FOR PERIODIC INTRODUCTION OF ELECTRONS INTO AN ELECTRON ACCELERATOR Filed Sept. 23, 1948 3|? M 7 Y J1 )i -1 i ""1 an W Fig.3. 21

i atented Mar. 27, 1951 CIRCUIT FOR PERIODIC INTRODUCTION OF ELECTRONS INTO AN ELECTRON ACCEL- ERATOR Rolf Wideroe, Zurich, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & (lie, Baden, Switzerland, a joint-stock company Application September 23, 1948, Serial No. 50,868 In Switzerland September 23, 1947 7 Claims.

This invention relates to'devices for accelerating a stream of charged particles such as electrons and the like and more particularly to an improved circuit arrangement by which the electron stream is introduced into the device at the proper instant relative to the forces under which the stream is accelerated.

The circuit is especially suited to electron accelerators of the betatron type (these are also known as ray transformers) and will be described in its relation thereto. In the betatron type of accelerator, where electron acceleration takes place along a circular orbit in an evacuated toroidal tube under the combined action of inducing and guiding magnetic fields, satisfactory operation requires the electron stream to be introduced periodically into the tube from an electron injector or gun at or near the instant when the magnetic field starts to build up from its zero value. Also it is advantageous to place the cathode of the injector under voltage only for a very brief instant so that electron injection will be equally as brief.

For synchronizing electron injection with the magnetic field of the device, it has been found desirable to control the starting of the cathode voltage from the energizing current for the magnetic field winding. One practical method has been to insert a saturated current transformer in the energizing circuit of the electron accelerator, with the primary being connected in series with the field Winding or in series with the condensers delivering the excitation current of the field winding. Because the current in the secondary winding of the current transformer is very small the magnetization of said peaking transformers will rise from zero to its maximum saturation value in a very short time after the current flowing through its primary winding passes through its zero value. This produces a brief peaked voltage in the transformer secondary which voltage can be used itself to directly excite the cathode of the electron injector or indirectly by applying the same to trigger the control grid of a tube such as a thyratron, high vacuum triode or the like whose plate-cathode elements are connected in circuit with the injector cathdode. In either case, it is essential that the voltage peak rise most rapidly, i. e. within a few microseconds and at the correct instant relative to the main magnetic field of the electron accelerator.

The present invention also makes use of a peaking transformer type of control circuit for energizing the cathode of the electron injector and has for its primary object the provision of a novel arrangement by which the starting instant of the voltage applied to the cathode of the electron injector can be finely adjusted as may be required to obtain the desired exact coincidence between electron injection and the magnetic field of the accelerator which varies periodically between zero and maximum values.

In particular, the desired result is attained by means of an additional current passed through the peaking transformer, the added current being displaced about 90 in phase relative to the current which energizes the magnetic field producing winding of the accelerator, and the impedance of the circuit which carries the additional current being so great that the inphasecurrent fiowing in this parallel connected circuit has practically no adverse effect upon the voltage produced by the peaking current transformer.

Several practical embodiments of the new control circuit are illustrated in the accompanying drawings. Fig l, a view in vertical central section through an induction type electron accelerator of conventional construction, shows one arrangement where the secondary of the peaking transformer is connected directly to the oathode element of the electron injector from which streams of electrons are introduced into the annular chamber to be accelerated; Fig. 2 illustrates in a more schematic maner a modification of the Fig. 1 circuit in which the additional current applied to the peaking transformer is obtained from a voltage source displaced 90 in phase relative to the voltage which energizes the field winding of the accelerator rather than from the latter as is done in Fig. 1, and Fig. 3 also a schematic view illustrates still another modified embodiment in which the voltage produced at the peaking transformer is used to trigger thyratron valves which in turn switch the cathode of the electron injector in and out relative to its energizing voltage.

Referring now to the structure and circuit shown in Fig. l, numeral l designates the winding of the accelerator, for instance a betatron, which when energized produce the accelerating and guiding magnetic field components for the injected stream of electrons. A typical betatron accelerator comprises a magnetic field structure made up from vertically disposed steel laminations of appropriate contour which includes a pair of cylindrical poles I a, lb separated by an air gap I0 and located concentrically along axis r-a:, and a pair of mutually confronting annular guide poles Id, le'separated by air gap If. Yoke members lg complete the magnetic circuit for the time-varying magnetic flux set up in the annular and cylindrical poles, under the influence of which, an electron stream will be accelerated around and around the tube along orbit Ic. Poles la, lb and Id, l e are surrounded by the annular winding l which is divided into upper and lower sections connected in series. An annular evacuated tube lh, preferably of glass, rests in the saturated peaking current. transformer 3- are grouped in series, and this.series group is connected in parallel with the winding 1.

across the secondary of transformer 3 andan anode I. A target anode l7 within the tube is struck by the electron stream emitted from cathode fi afte'r the: streamhasbeen fully; accelerated onits orbitals.

If more: detailed" information is desired concerning the: actual construction of. the windin the magnetic structure on which it is wound or the arrangementrofthe electron injector and evacuated tube etc., reference-may be had to mycofendingapplication, Serial No. 720,544; filed January: 7; 1947and issuedJanuary 16; 1951 as Patent: No. 2,538,718.-

In accordance. with the invention,theaddi' tionalcurrent bywhich the-starting moment of their peaked voltage: wave produced by transformer 3 can be'nshiftedis' supplied by a transformer 9. The primary 9a of this transformer isconnected'across the voltage line 11. and the voltageproducedby-the secondary 9b is connected through an adjustable resistance 8: across the transformer primary 4; Because of the fact that the fieldwinding: i possesses a'highly inductive characteristic, its energizing current and hence the main. current through primary-winding s will lag; almost 90 behind the applied voltage u. The additionalcurrentv that flows through resistance 8' and" transformer winding 4 and which is also derived from the same voltagesource-u will likewise bedisplaced in phase by about-90 relativeto the energizing currentfor Winding; l-- and transformer winding 4; by'adjustingthe'resistoraa, the phase position of the resultant current-flowing through transformer winding: 5 and which determines the starting of the peakedvoltage wave produced acrossthe output terminals of the transformer secondary iican be very conveniently adjusted.

The amplitude "of theqpeaked-voltage wave is somewhat diminished bythe additional currentin phase with the current through the field winding passed through the primary 3 because of the presence oftheparallel connected resistor 8.

If C ischosen .to represent the resulting capacity, i. e; the capacity of the windingsplus the capacity of the connected circuit with the electron injector 6l;.andL the inductance of transformer 3 (both values being referred to the winding 4 through which the additional current flows), the maximum value of the voltage peak becomes:

so long as the parallel resistance 8 (designated R in the aboveequation) is still large. As long as this parallel resistance The elec-' tron injector located within tube lh is comprised of an electron emissive cathode 6- connected Thus practical arrangement is shown in 2. Here,

it will be observed that the windin I, transformerSand condensers 2 are connected in the same mannera s in Fig. 1. For energizing Winding. I" and' producing the additional current to be applied to peaking transformer ii a three phase voltage supply. sourceisutilized- The separate phases are indicated by terminal vectors R,-- S and T andthese-arestar connected tea -common:- voltage point -O Winding i is connected acrossterminalsR and- S,-and-- transformer I8 whichis-toxsupplythe additional current for peaking transformer 3 has its primary lilaconnected across: terminals O-T, the voltage across the latter being of course displaced in phase relativeto that across terminals RS. The voltageacross the secondary winding l lib -is made adjust able and .appliedacross-the primary winding. 4' with a choke coil l2-inthe.circuit. Operation of the Fig. 2- controlis otherwise the same as in- Fig. 1.. So far as diminution ofthevoltage peak is concerned, the presence of choke. coill2. in parallel with-windings will cause a reduction in;the amplitude of the voltage peak in the ratio Lpar 5+ L'parto the capacity of the peaking transformer 31. and, in order to keep the total capacity C small,

the choke coil l2. must therefore be provided with a low-capacity type of winding of not \too greata-numberofturns. The Fig; 2- circuit is preferable to' the Fig. i-arrangement in that the wattage losses for the additional current are less forcoil l2-than for resistances.-

Also the'wattless power in the choke will be small and can therefore be disregarded. Instead of achoke 12-. to establish a current through transformer winding 4 which-lags the current through winding I by 90, a condensercould be used and thus establisha 90 leading current but if this bedone, the Capaoity of the condenser must be small -in-relation to the transformer capacity C to prevent a decreasing effect on the peakvoltage, and for this-reason, therequired-voltage across. the condenser would .have. to beinconveniently high in orderto produce the additional current required in thetransformer primary i. Thus from practicalconsiderations; the use of achoke coil producing the additional current at a 90 lag in phaserelativeto th'emagnetiz ing. current of the acceleratoris to be preferred inmost cases.

Fig. 3 illustrates arr-embodiment of the invention wherein the voltage produced in the secondary winding of the peaking transformers also be takenused' indirectly to effect brief ''nergization of the cathode of the electron injector. In this form of the invention, the betatron (as described in more detail in my aforesaid application), is adapted to accelerate separate electron streams on both positiveand negativehalves of the alterhating voltage wave u, the electron streams being injected into'the tube alternately and in opposite directions at the beginning of each positive and negative energization of the field winding I. Thus two electron injectors operating 180 apart in phase are provided and each has its own peaking transformer type of control. Referring now to Fig. 3, it will be seen that the winding I and condensers 2-2' are arranged as before. However, two peaking transformers 3 and 3' in series 7 with condensers 2 are required, as well as two other transformers 9 and 9' which serve as the source of supply voltage for the additional current that is applied to the respective primary windings of the transformers 3 and 3. Resistances 8 and 8, both adjustable, are inserted in the secondary circuits of transformers 9 and 9' to control the magnitude of the respective additional currents put through transformer primaries 4 and 4. Operation of the Fig. 3 circuit thus far is the same as in Fig. 1 with the additional currents supplied by transformers 9 and 9 being displaced 90 in phase from the currents passed through the main field winding I.

In the circuits shown in Figs. 1 and 2, the secondary of the peaking transformer is connected directly to the cathode of the electron injector. In Fig. 3, a modified arrangement is utilized, in which a thyratron type of tube connected in the circuit between the cathode of the electron injector and a source of voltage is triggered at the proper instant by the peaked voltage produced in the secondary of the peaking transformer. The energizing circuit for the cathode 6 of the electron ejector producing electron stream 20 includes a diode rectifier I8 connected across condenser 2 through resistor I9, a thyratron tube I4, inductance I! and condenser I6. The energizing circuit for cathode 6' of the electron injector producing electron stream 20' includes diode rectifier I8 connected across condenser 2 through resistor I9, thyratron tube I4, inductance I1 and condenser IS. The grid circuit of thyratron I4 includes a fixed biasing voltage represented by battery l3 and the secondary 5 of peaking transformer 3; similarly the grid circuit of thyratron I 4' includes battery l3 and the secondary 5 of peaking transformer 3'.

Operation of the Fig. 3 circuit is as follows:

When the voltage peak occurs in transformer secondary 5, thyratron I 4 is triggered to a conductive condition thus connecting the voltage on condensers 2 and 2' through thyratron I4 and rectifier I8 to an oscillatory circuit comprised of condenser I6 and inductance IT. This effects a peaked voltage of brief duration on condenser I6, the voltage rising according to the equation /2u 1 cos wt) where 11 is the frequency of the oscillatory circuit I6-II, u is the voltage applied to winding I and the losses are small. Such voltage is of course likewise applied to cathode 6 and produces the electron stream 20. Substantially 180 later in the wave of alternating voltage u, thyratron tube I4 is triggered thus connecting the voltage on condensers 2 and 2' through thyratron I4 and rectifier I8 to an oscillatory circuit composed of condenser I6 and inductance I1; and the peaked voltage produced across condenser l 6' is similarly impressed upon cathode 6' to produce the other and oppositely directed electron stream By adjusting the resistors 8 or 8, the voltage peaks produced by transformers 3 or 3' and hence the triggering instants of thyratrons I4, I4 can be advanced or retarded as may be desired to thereby adjust the instant of electron injection relative to the varying magnetic field produced by the winding I.

In conclusion, it is to be understood that while the above described constructions represent preferred forms of the invention, still other changes and modifications are possible without departing from the spirit and scope of the appended claims.

7 Thus for example, while the additional, adjustable out-of-phase current is illustrated as being applied directly to the primary winding 4 of the peaking transformer, such is not the only way of obtaining the desired result since the additional current could just as well be applied to a separate winding 0n the transformer core.

I claim:

1. In a device for accelerating charged particles such as electrons and the like to high velocity, an electron injector including an electron emissive cathode, means including a main winding for cyclically producing a time-varying magnetic field of such space distribution as to confine the electrons emitted from said injector to a desired orbit while continuously accelerating them along such orbit, a peaking transformer for periodically producing in its secondary a peaked voltage effective to briefly energize said cathode to thereby produce electron streams periodically, said transformer being provided with a primary winding which is magnetized by the current flowing through said main winding and means providing said transformer with an additional magnetizing current phase-displaced relative to said first magnetizing current and impedance means in the circuit of said additional current of such a value that the peak voltage in the secondary of said transformer is not appreciably reduced by said additional current circuit.

2. A device for accelerating charged particles as defined in claim 1 wherein said transformer has but one primary winding and both of said magnetizing currents are applied to such wind- 3. A device for accelerating charged particles as defined in claim 1 wherein said additional current is produced by a voltage in phase with the voltage applied to said magnetizing winding of the accelerator and the additional current circuit includes an adjustable resistance.

4. A device for accelerating charged particles as defined in claim 1 wherein said additional current is produced by a voltage which is displaced in phase relative to the voltage applied to said magnetizing winding of the accelerator, and the additional current circuit includes a reactance.

5. A device for accelerating charged particles as defined in claim 4 wherein said reactance is mainly inductive in character.

6. A device for accelerating charged particles as defined in claim 1 wherein said cathode is connected directly to the secondary of said peaking transformer.

7. A device for accelerating charged particles as defined in claim 1 wherein the energizing circuit for said cathode includes a grid controlled tube for rendering said cathode circuit conductive and non-conductive, and the secondary of i 8 saidpeaking transfcrmer is-c0nnected in$he-g1id Number Na-me Date cir cilit 0f said .tube. 7 2,297,305 Kerst ..,-Sept.29,-1942 tRQLF 36 2,394,070 Kerst Feb. 5, 1946 2,394,071 Westendorp Feb.'5, .1946 REFERENCES CITED 5 OTHER. REFERENCES The following references are of record in'the file 70f thl's patent: atorsflJoumal of Applied Science, vol. '18,,pp..-1-

UNITED STATES PATENTS 13, Jan. 1947.

Number Name Date 10 2,059,863 Hansen -NOV.-3, 1936 Kaiser: European Elec'tronlnduction Accelen 

